1. Technical Field
The present disclosure relates to strip-line circuitry. More particularly, the present disclosure relates to printed circuit boards including strip-line circuitry and methods of manufacturing the same.
2. Discussion of Related Art
Commonly used techniques for circuit and/or system interconnect include microstrip and strip-line transmission lines. In a simple representation, as shown in FIG. 1, a microstrip transmission line consists of a conductive trace 13 of controlled width WC disposed on a low-loss dielectric 11 which is, in turn, disposed on a ground-plane layer 12. In microstrip there is one ground plane, while in strip-line, there are two.
Microstrip and strip-line transmission lines can be fabricated using printed circuit board technology. Printed circuit boards (PCBs), sometimes referred to as printed wiring boards (PWBs) or etched wiring boards, are widely used in the assembly of discrete electrical components into operating circuits. PCBs generally provide a reliable and economical means of interconnecting electrical signals between system components. PCBs are available in a variety of different types and may be classified in a variety of ways.
PCBs are generally used to mechanically support and electrically connect electronic components using electrically-conductive pathways or signal traces that conduct signals on the PCB. A typical PCB includes one or more layers of insulating material upon which patterns of electrical conductors are formed. In addition to a pattern of conductive traces on the PCB, a patterned array of metal-filled through-holes, or vias, may be formed to allow for layer-to-layer interconnections between various conductive features.
PCBs may be classified as single-sided PCBs, double-sided PCBs, and multi-layer PCBs, according to the number of circuit pattern surfaces. Microstrip transmission lines, for example, are commonly fabricated on double-sided PCBs. PCBs may include circuits that perform a single function or multiple functions.
A typical PCB may include a variety of electronic components. Electronic components form parts of electronic circuitry and may be classified in a variety of ways. An electronic component may be classified as active or passive. In general, an active component is any type of circuit component with the ability to electrically control the flow of electrons or other electrically-charged particles. Some examples of active components are transistors, integrated circuits (ICs), silicon-controlled rectifiers (SCRs), and triodes for alternating current (TRIACs). Components incapable of controlling current by means of another electrical signal are generally classified as passive components. Examples of passive components include capacitors, resistors, inductors, transformers, and diodes. A PCB on which electronic components are mounted is sometimes referred to as a printed circuit assembly (PCA) or a printed circuit board assembly (PCBA).
In some circuits, such as high-frequency circuits, e.g., microwave circuits, maintaining controlled impedance across the PCB may be required in order to achieve consistent electrical performance, e.g., in terms of amplitude and phase response. A variety of PCB trace geometries are possible with controlled impedance designs. A two-sided PCB design wherein a planar conductor line is spaced above a ground plane, as shown in the cross-sectional view of FIG. 1, can be designed for controlled characteristic impedance. This geometry is known as a surface microstrip, or simply microstrip. In microstrip the planar conductor lines are usually formed by chemically etching away unwanted areas of material, e.g., metal, from a conductor layer, such as copper.
The impedance of a planar conductor in a microstrip transmission line format is determined by factors such as the dielectric characteristics of the surrounding materials, the width of the conductor line and its spacing from the ground-plane layer, among other things. In the surface microstrip configuration the signal conductor is exposed to air, so the effective dielectric constant impacting the impedance of the conductor is a combination of the relative dielectric constant, ∈r, of the PCB dielectric substrate as well as that of the air above the circuit. Typically, the effective dielectric constant will be somewhere between 1 (∈r of air) and about 4 (∈r of FR-4 substrate).
An approximate expression of the characteristic impedance Z0 of a microstrip transmission line, as shown in FIG. 1, is given by Equation 1 (below) and is expressed in Ohms (Ω). In Equation 1, the measurement unit is mils, i.e., 1 mil=0.001 inches.
FIG. 1 shows a microstrip transmission line 10 that includes a signal trace 13 on the top side of a PCB dielectric substrate 11 and a ground (or power) plane 12 on the bottom side of the substrate 11. Using Equation 1, for the signal trace 13 of width WC and thickness TC, separated by distance TD from the ground plane 12 by the PCB dielectric substrate 11, the characteristic impedance Z0 of the microstrip line 10 may be expressed as
                                                        Z              0                        ⁡                          (              Ω              )                                ≈                                    87                                                                    ɛ                    r                                    +                  1.41                                                      ⁢                                                  ⁢                          ln              ⁡                              [                                                      5.98                    ⁢                                          T                      D                                                                            (                                                                  0.8                        ⁢                                                  W                          C                                                                    +                                              T                        C                                                              )                                                  ]                                                    ,                            (                  Equation          ⁢                                          ⁢          1                )            where ∈r is the dielectric constant of the PCB substrate 11. Equation 1 is generally valid when 0.1<WC/TD<2.0 and 1<∈r<15.
Microstrip transmission line operation may be impaired by stray electromagnetic coupling between the line conductor and nearby objects. In microstrip, the line conductor is coupled to the ground plane below, which reduces EMI (electromagnetic interference) by absorbing some of the electromagnetic field lines. Fringing of the electromagnetic fields that extend above the line conductor to foreign objects may introduce irregularities into the impedance and velocity factor of the line, with a resultant negative effect on circuit performance. To mitigate the effects of electrical field fringing, additional constraints may be imposed, e.g., requiring the width of the ground plane be such that it extends past each edge of the signal trace by at least the width of the signal trace.
Strip-line transmission line construction, as shown in the cross-sectional view of FIG. 2, is generally characterized by a planar conductive trace 23 sandwiched by dielectric material 21 and disposed between two ground or power planes (commonly referred to as reference planes). An electric field 46 is generated between the conductor line 23 and the upper and lower ground layers 24 and 22, and a magnetic field 58 is generated around the conductor line 23. In strip-line routing, the second ground (or power) plane, which is omitted in microstrip construction, shields the conductor from the effects of nearby objects and serves to confine the electromagnetic fields to the region between the two reference planes.
In strip-line routing, all of the electromagnetic field lines are coupled to the upper and lower reference planes, and the chance of stray coupling between the line conductor and nearby objects is avoided. In addition to minimal radiation losses, a strip-line circuit's upper and lower ground planes may form a more efficient ground return path with less surface resistance than in the microstrip configuration.
An expression of the characteristic impedance, Z0sym, of a symmetric (balanced) strip-line, as shown in FIG. 1, is given by Equation 2. Using Equation 2, for the signal trace 23 of width WC and thickness TC, sandwiched by the PCB dielectric 21 of thickness TD disposed between the reference planes 22 and 24, the characteristic impedance Z0sym of the symmetric strip-line 20 can be expressed as
                                                        Z                              0                ⁢                sym                                      ⁡                          (              Ω              )                                ≈                                    60                                                ɛ                  r                                                      ⁢                                                  ⁢                          ln              ⁡                              [                                                      4                    ⁢                                          T                      D                                                                            0.67                    ⁢                                          π                      ⁡                                              (                                                                              0.8                            ⁢                                                          W                              C                                                                                +                                                      T                            C                                                                          )                                                                                            ]                                                    ,                            (                  Equation          ⁢                                          ⁢          2                )            where ∈r is the dielectric constant of the PCB dielectric 21. Equation 2 is generally valid for the following dimension ratios: WC/(TD−TC)<0.35 and TC/B<0.25.
Microstrip and strip-line technologies are most commonly used routing configurations in circuits and can be used to provide well-characterized transmission line conductors that can be used to interconnect discrete circuit elements and to perform various impedance transformation functions. Strip-line transmission lines offer some electrical performance advantages compared to microstrip, but strip-line is more complex to fabricate than microstrip. In high-frequency circuits, e.g., microwave circuits, cost and/or space savings may be attained by using strip-line technology.